Method for tone adapting an image to a target peak luminance LT of a target display device

Abstract

An image processing method is described. The method includes obtaining low dynamic range expansion exponents ELDR(p), obtaining target expansion exponents ET(p) as a weighted sum of the high dynamic range expansion exponents EHDR(p) and of the low dynamic range expansion exponents ELDR(p), applying obtained target expansion exponent ET(p) to low dynamic range luminance values YLDR of a low dynamic range version of the image, resulting in target luminance values YT, and building a tone-adapted version of said image based on said target luminance values YT.

The invention claimed is: 
     
       1. An image processing method for tone adapting an image to a target peak luminance L T  of a target display device from a low dynamic range version of this image adapted to be displayed on a display device having a low peak luminance L LDR , and from high dynamic range expansion exponents E HDR (p) to be applied to low dynamic range luminance values Y LDR  or to low pass filtered luminance values of said low dynamic range version of said image to get expanded luminance values Y HDR  of a high dynamic range version of said image adapted to be displayed on a display device having a high peak luminance L HDR , said method comprising:
 obtaining low dynamic range expansion exponents E LDR (p) such that, when applied to low dynamic range luminance values Y LDR  or to low pass filtered luminance values of said low dynamic range version of said image, low dynamic range luminance values Y LDR  are linearly scaled within a high dynamic range [0, L HDR ], 
 obtaining target expansion exponents E T (p) as a weighted sum of the high dynamic range expansion exponents E HDR (p) and of the obtained low dynamic range expansion exponents E LDR (p), with a weight 0≤α in ≤1 of the high dynamic range expansion exponent E HDR (p) and with a weight of the low dynamic range expansion exponent E LDR (p) equal to (1−α in ), wherein the weight α in  of the high dynamic range expansion exponent E HDR (p) is proportional to a difference between the target peak luminance L T  of the target display device and the low peak luminance L LDR , this difference being scaled by another difference between the high peak luminance L HDR  and the low peak luminance L LDR , 
 applying obtained target expansion exponent E T (p) to low dynamic range luminance values Y LDR  or to low pass filtered luminance values of said low dynamic range version of said image, resulting in target luminance values Y T , 
 building a tone-adapted version of said image based on said target luminance values Y T . 
 
     
     
       2. An image processing device for tone adapting an image to a target peak luminance L T  of a target display device from a low dynamic range version of this image adapted to be displayed on a display device having a low peak luminance L LDR , and from high dynamic range expansion exponents E HDR (p) to be apply to low dynamic range luminance values Y LDR  or to low pass filtered luminance values of said low dynamic range version of said image to get expanded luminance values Y HDR  of a high dynamic range version of said image adapted to be displayed on a display device having a high peak luminance L HDR , said device comprising at least one processor configured for:
 obtaining low dynamic range expansion exponents E LDR (p) such that, when applied to low dynamic range luminance values Y LDR  or to low pass filtered luminance values of said low dynamic range version of said image, low dynamic range luminance values Y LDR  are linearly scaled within a high dynamic range [0, L HDR ], 
 obtaining target expansion exponents E T (p) as a weighted sum of the high dynamic range expansion exponents E HDR (p) and of the obtained low dynamic range expansion exponents E LDR (p), with a weight 0≤α in ≤1 of the high dynamic range expansion exponent E HDR (p) and with a weight of the low dynamic range expansion exponent E LDR (p) equal to (1−α in ), wherein the weight α in  of the high dynamic range expansion exponent E HDR (p) is proportional to a difference between the target peak luminance L T  of the target display device and the low peak luminance L LDR , this difference being scaled by another difference between the high peak luminance L HDR  and the low peak luminance L LDR , 
 applying obtained target expansion exponent E T (p) to low dynamic range luminance values Y LDR  or to low pass filtered luminance values of said low dynamic range version of said image, resulting in target luminance values Y T , 
 building a tone-adapted version of said image based on said target luminance values Y T . 
 
     
     
       3. An electronic device incorporating the image processing device according to  claim 2 . 
     
     
       4. The electronic device according to  claim 3  selected from the group consisting of a camera, a TV set, a monitor, a head mounted display, a set top box, a gateway, a smartphone and a tablet. 
     
     
       5. A non-transitory storage medium carrying instructions of program code for executing steps of the method according to  claim 1 , when said program is executed on a computing device.